Support chaining

Have you through of adding chaining to `subscribe`, `once` and `remove` methods?

E.g.

``` js
Mediator
.subscribe('channe:one', doOne)
.subscribe('channel:two', doTwo)
.subscribe('channe:three', doThree)
```

I notice `subscribe` and `once` return the `subscriber`. What is the use case for this?

If this would be a positive change I'd be happy to submit a pull.